The beneficial effects of estrogen on bone maintenance, blood lipid profile, and the cardiovascular system are well known and account for the widespread use of hormone replacement therapy (HRT) in postmenopausal women (1). Estrogens and anti-estrogens affect several tissues, and the pattern of effects observed depends upon the particular ligand used (2). A major advance toward understanding the differential effects of various estrogenic compounds came with the recent discovery of an additional form of the estrogen receptor (3). The newly discovered receptor, named ER-β, is similar in sequence to the previously known form, now called ER-α. Mapping the distribution of ER-β and ER-α mRNA in normal and neoplastic tissues has provided an intriguing picture of differential expression patterns in different tissue types (4,5,6,7). The existence of clear-cut differences in receptor expression suggests that tissues could be targeted selectively with ligands selective for ER-α or ER-β.
Like all known nuclear receptors, estrogen receptors function as ligand-activated transcriptional factors and have a modular structure consisting of six discrete domains, named A-F. These domains mediate binding to DNA, ligands and co-activators (8,9,10,11). The E domain of ER-α binds ligands such as 17β-estradiol and the phytoestrogen, genistein. The E-domains of ER-α and ER-β are 59% identical in sequence and have a predicted mass of approximately 25 kD. The natural ligand, 17β-estradiol, binds both with similar affinity. In contrast, genistein is selective, having 30 fold greater affinity for ER-β than for ER-α ((3) and H. Harris, unpublished observations).
The ligand binding domains (LBDs) of all studied nuclear receptors change conformation substantially upon ligand binding (12,13,14,15), particularly in the positioning of helix 12 (H12). In the case of ER-α, the position of H12 induced by the ligand depends on whether the ligand is an agonist (estradiol or diethylstilbestrol (DES)) or antagonist (raloxifene or tamoxifen). In the agonist complex, H12 packs against helices H3, H5, H6 and H11, forming a lid over the ligand. In this complex, H12 forms a wall perpendicular with and at one end of the co-activator binding groove formed by residues in H3, H4, H5 and the turn between H3 and H4. Peptides derived from the NR box II region (16,17,18,19) of the co-activator, GRIP1 can bind in this groove (11), suggesting this is an important aspect of transcriptional regulation. In contrast, steric hindrance from a bound antagonist displaces H12 so that it now binds in a hydrophobic groove formed by residues from helices 3 and 5. In this position, H12 binds to and occludes the co-activator recognition site, mimicking the interactions formed by the NR box II with the LBD and probably preventing modulation by co-activators. From these results it is clear that the structure of the bound ligand affects the overall structure of ER-α and its interactions with co-activators.